In recent years, solar cells attract attention as energy means to replace petroleum energy, which is a cause of global warming, and are increasingly demanded. A solar cell is a solar power generation system that converts the energy of sunlight directly into electricity, and the heart of the solar cell is formed of semiconductors. The structure of the solar cell is such that a solar cell element is not used solely as it is. Generally, several to several tens of solar cell elements are unitized by wiring them in series or in parallel and packaging them in various manners to protect the elements over a long period of time (for 20 years or longer). The unit incorporated in the package is termed a solar cell module, which generally has a structure where: the side of the solar cell module that receives sunlight is covered by glass; gaps are filled with a filling material formed of a thermoplastic resin; and the back side is protected by a protective sheet termed a back sheet and having a multilayer structure formed of a heat resistant and weather resistant plastic material and the like.
In the case of, for example, a solar cell module using thin-film silicon, the back sheet generally has the lamination structure of, starting from the solar cell element side, a polyester film/an adhesive/a moisture-proof layer such as a metal foil or a film including a metallic thin-film layer/an adhesive/a polyvinyl fluoride film or a polyester highly durable moisture-proof film (the outermost layer).
To improve the conversion efficiency of the solar cell, the solar cell elements and the back sheet for solar cells are modified. For example, even when light incident from the front side of the solar cell module has transmitted through the solar cell elements, the light is made incident on the solar cell elements again by reflecting it. Such an efficient use of the light makes it possible to increase photovoltaic power.
Accordingly, the back sheet for solar cells requires a high reflectance, and also requires a low reflection directivity in order to uniformly reflect light to nearby elements. Further, crystalline silicon to be used as a solar cell element has a sensitivity in the wavelength range of from 400 to 1200 nm, and therefore, has a high sensitivity in the near-infrared region rather than in the visible light region. Thus, the reflection properties of the back sheet for solar cells are important not only in the visible light region but also in the near-infrared region (wavelengths of from 700 to 1200 nm). In response, as a polyester film to be used for the back sheet for solar cells, the following are proposed, for example.
Patent Document 1 proposes a film for sealing the back side of a solar cell, which improves the reflection efficiency by mixing a polyethylene terephthalate resin with an incompatible resin and using air bubbles generated during the manufacturing of the film and exemplifies a film for sealing the back side of a solar cell, having a reflectance of from 84 to 87% at a wavelength of 560 nm.
Patent Document 2 proposes a plastic film for a protecting film of the back surface of a solar cell, obtained by blending a white coloring agent in plastic forming a plastic film, and exemplifies a plastic film for a protecting film of the back side of a solar cell, having an average reflectance of from 82 to 93% in the wavelength range of from 600 to 1400 nm.
Patent Document 3 proposes a white polyester film for a protecting film of the back surface of a solar cell, in which a polyester resin contains barium sulfate particles and exemplifies a white polyester film for a film protecting film of the back surface of a solar cell, having an average reflectance of from 86 to 97% in the visible light region of wavelengths of from 400 to 700 nm.
Patent Document 4 proposes a solar cell module using: a biaxially stretched polyethylene terephthalate film kneaded with a white pigment; a polyethylene terephthalate film containing fine air bubbles; or a resin film including a coating film layer having light reflectivity and exemplifies a back cover film having a total light reflectance of 95% and 97%.